Fiber reinforced thermoplastic composite components (or parts) have a high strength-to-weight ratio or a high stiffness-to-weight ratio and desirable fatigue characteristics that make them increasingly popular as a replacement for metal in a wide variety of applications. Such components have found expanding applications in, for example, the aerospace, automotive and wind energy industries.
Conventional methods of fabricating thermoplastic composite components include autoclave and hot-press forming. In autoclave processing, prepreg fiber sheets of continuous, woven or chopped reinforcing fibers are formed with an uncured matrix resin impregnated therein. The prepreg sheets are placed (or laid-up) by hand or with fiber placement machines directly upon a tool or die having a forming surface contoured to the desired shape of the completed part, or are laid-up in a flat sheet which is then draped and formed over the tool or die to the contour of the tool. Then, the resin in the prepreg layup is consolidated (i.e., pressed to remove any air, gas, or vapor) and cured (i.e., chemically converted to its final form usually through chain-extension) in a vacuum bag process in an autoclave (i.e., a pressure oven) to complete the part. Autoclave processing is time consuming because of the need for vacuum bagging.
In hot press forming, a prepreg is laid-up, bagged (if necessary) and placed between matched metal tools that include forming surfaces that define the internal, external, or both mold lines of the completed part. The tools and composite prepreg are placed within a press and then the tools and prepreg are heated under pressure to produce a consolidated part. Hot press forming requires high molding pressures of about 1,500 psi, molding temperatures below the melting point of the prepreg to permit unloading in a timely manner, and lower percentages of fiber reinforcements in the prepreg. Lower molding temperatures provide limitations on the strength and stiffness of the component and restrict how thin the walls of the composite components may be.
It is also known to consolidate and mold thermoplastic composite components from a pre-form using an inductively heated consolidation tool, as disclosed in commonly-owned U.S. Pat. No. 8,017,059, the disclosure of which is incorporated by reference herein. In general, a pre-form is placed between first and second electrically conductive tooling dies or molds (usually metal), which are inductively heated and compressed to form a molded thermoplastic composite component. Induction heating is a process in which the mold is heated by electromagnetic induction. During such heating, eddy currents are generated within the metal and the electrical resistance of the metal leads to Joule heating thereof. An induction heater typically comprises an inductor through which a high-frequency alternating current is passed. A susceptor may be used in or adjacent to the pre-form to achieve the necessary heating for consolidation or forming. The susceptor is heated inductively and transfers its heat principally through conduction to the pre-form sandwiched between opposing susceptor facesheets.
Because of the expanding industrial applications for thermoplastic composite components, there is a need to improve processing techniques and facilities to enable widespread manufacture and use of such parts. In particular, there is a need for processes that will enable rapid heating, consolidation, molding and cooling in a controlled manner to increase production rates and volume of lightweight and high quality thermoplastic composite components in an affordable and energy efficient manner.